Distributed optical resonator with thin receiver unit

ABSTRACT

A receiver for receiving an incident beam of optical power from a remote transmitter over a predefined field of view, comprising an input lens having a high durability coating that can withstand domestic handling and contamination. Such a high durability coating may reflect a non-insignificant part of the light incident thereon. Behind the lens, there is fitted a retroreflector disposed such that it reflects that part of the incident beam traversing the lens, back through the lens to the transmitter. Reflections from the front surface of the lens impinge on one or more transparent beam catchers appropriately located, and equipped with energy conversion devices, such as photovoltaic cells, to convert light from the reflections of the incident beam into electricity. Additional energy conversion devices may be located inward of the lens, to collect and convert reflections from the inner surface of the lens, of light returning from the retroreflector.

RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/548,175, filed Aug. 2, 2017, which is a National Phase applicationfiled under 35 USC § 371 of PCT Application No. PCT/IL2016/050120 withan International filing date of Feb. 2, 2016, which claims priority ofU.S. Patent Application 62/125,829 filed Feb. 2, 2015. Each of theseapplications is herein incorporated by reference in their entirety forall purposes.

FIELD OF THE INVENTION

The present invention relates to the field of optical power transmissionbetween a transmitter and a receiver, connected as complementary partsof a distributed optical resonator operating as a long laser, especiallyfor the purpose of transferring power to a thin hand-held mobileelectronic device from a centrally located transmitting unit.

BACKGROUND OF THE INVENTION

In prior art patent documents U.S. Pat. No. 4,209,689 to G. Linford for“Laser Secure Communications System”, WO 2007/036937 for “DirectionalLight Transmitter and Receiver”, WO 2009/083990 for “Wireless LaserPower Transmitter” and WO 2012/172541 for “Spatially Distributed LaserResonator”, the latter three having a common inventor with the presentapplication, there are described various distributed resonator lasersystems. All of the distributed resonator systems described in thatprior art include a transmitter which incorporates a gain medium and aretro reflector acting as one mirror of the lasing system, and areceiver which also incorporates a retro reflector which can act as thesecond mirror of the lasing system and an output coupler for extractingthe energy from the resonator.

In the case of WO 2012/172541, various optical elements are alsoincorporated to manipulate, monitor, and control the beam. Additionally,there is also taught therein elements such as beam blockers and irises,for blocking or avoiding unwanted reflections from the receivers. Suchreflections could be a potential safety hazard.

The receivers described in the prior art may not be optimum for use inmodern portable electronic devices, such as cellular phones and tablets,or even laptop PCs, as they are generally too large, and especially toodeep. Furthermore, they may be unsuitable for use in the consumerenvironment, since, given the power requirements of cellular phones andother portable electronics devices, which is reflected in the powerlevel circulating in the distributed laser cavity, they may not fulfilsafety regulations. This is particularly so in an environment wherefinger prints, dirt, spilled liquids and the like are a reality of life,and their presence on the surface of the light receiving element of thereceiver may compromise any safety measures taken to prevent stray laseremission.

Such systems can potentially be used to transfer energy to a remotedevice, such as a cellular telephone, and charge it. Eliminating orreducing the need to charge the phone by a cord, or by being placed onan inductive charging mat, therefore extends the time a device canoperate between connections to a wall charger or being placed on astatic charging mat.

As an example of the requirements of such a receiver, a receiverincorporated into the body of a cellular telephone, aimed at chargingit, would typically need to have the following limiting parameters:

-   (a) Thickness preferably less than 6 mm.-   (b) Radiation emissions limited, such that they are not a safety    hazard. Typically, they should be less than the levels specified in    the IEC60825 standard.-   (c) Capability of generating at least 1 W of power-   (d) A field of view of at least ±30 degrees, where the field of view    is the sum of all angular directions from which the receiver could    receive power, if a properly aligned transmitter were to transmit    power thereto.-   (e) The transmission range should be at least 3 meters.

Some of the above characteristics are more and some less critical. Thefirst two criteria are perhaps the most important. The thickness ismandated by the thickness of mobile electronic devices such as cellulartelephones. Unless the thickness is limited to that of these devices,typically 6 mm, the receiver solution may not be commerciallyacceptable. Therefore it is important to provide receivers sufficientlythin for inclusion within the thickness of such portable devices, wherethe field of view of the receiver faces in a direction generally towardsthe normal to the large surface of the device, since that is theposition in which such devices are usually held. With regard tocriterion (b), the front surface of the optical input element of suchdevices, on which light impinges before entering the receiver, is aparticularly problematic feature, since reflections from that frontsurface may be a main source of safety problems with the receiver.Therefore, it would be important to provide an optimized coating for thefront surface of such a receiver input element, to prevent or reducesuch reflections to meet the relevant safety standard maximalpermissible exposure (MPE). Furthermore, the optical beam blockingarrangement used should be such as to prevent unwanted reflections abovethe MPE from being directed in directions other than back to thetransmitter unit.

Such receivers typically consist of a lens and a partially reflectivemirror at its focal plane, such as can be seen in FIG. 7 of the abovereferenced publication WO 2012/172541. The front surface of the lensreflects some of the light impinging upon it, and such reflection can becontrolled and minimized but cannot be completely eliminated. Suchsystems are limited in the amount of power they could safely transmit,as some of the power will always be reflected by the front surface. Insuch prior art systems, this surface is coated with an anti-reflection(AR) coating for at least some of the following reasons:

-   1. Safety—Optimizing the system for higher power transmission    requires very low reflection from the front surface so that it would    not exceed the maximal permissible exposure (MPE), such as that    mandated by IEC60825. This would require having the front surface    coated with anti-reflective (AR) coating, reflecting as little as    possible light of the incoming light and therefore creating the    least hazard.-   2. Increasing efficiency and usefulness—Such AR coating would also    improve the overall system efficiency. Reflections from the front    surface represent a power loss, and losses reduce the system's    efficiency dramatically as they compete with output coupling. To    achieve minimal losses, the front surface should have a coating    having a reflection as low as possible.-   3. Increasing field of view —since symmetrical receiver    configurations create the widest field of view, as they are    indifferent to the direction the beam comes from, the front surface    of the input lens of the receiver should be a spherical or nearly    spherical surface. The radius of curvature of such a surface should    be chosen to be minimal as this would serve to make the reflected    beam intensity decrease rapidly with distance from the receiver as    the beam diverges, so that it poses reduced risk after a short    distance.

For a receiver with a large field of view, there are additional specificadvantages for the use of such an AR coating:

-   (a) Without an AR coating, such a surface would have different    Fresnel reflection/transmission properties at different angles and    this may distort the beam's wavefront.-   (b) The large field of view makes it difficult to use other means    such as beam blockers to block the light reflected from the front    surface. Such physical beam blockers are simpler to design when the    field of view is small.

However, the use of an AR coating on a domestically used device such asa cellphone is problematic, since in such a consumer environment, dirt,spilled liquids, dust, fingerprints, and similar layers on top of the ARcoating will amend the effectiveness of the AR coating, leading tohigher reflections, and secondly, may eventually lead to degradation andpeeling of the AR coating, again compromising safety. Therefore,alternative solutions must be found in order to reduce potentiallydangerous reflections from the input optical surface of the receiver.

Other alternative solutions have been proposed, but each of thesesolutions has its own disadvantages:

-   1. Increasing the beam diameter so much that reflection from the    receiver lens would be at a size and intensity so that it would be    safe. Typically, a beam diameter of more than 7 mm would be    required, which is the standard aperture used in IEC60825. However,    using this technique, the size of the beam needed to transmit the    power required by a mobile device may be bigger than the entire area    available for a receiver on the mobile device! For example, to    supply power in the range of between 1 and 5 W, as needed to charge    cellphones of various sizes, and using 20% photovoltaic efficiency,    an optical power in the range of at least 5 to 25 W is needed.    Taking into account that an uncoated glass surface reflects about 4%    of the power incident on it, the range of optical power that needs    to be transmitted ranges from at least 5.2 to 26 W. The MPE for 1400    nm light according to the IEC60825 (2^(nd) edition) is of the order    of 40 mW/cm², Taking into account the field of view required (at    least ±30 degrees) a beam having a diameter of approximately 3 cm    would be required to charge the phone. Such a large beam cannot be    input through the surface available for that purpose in a modern    cellphone.-   2. Using a beam block, typically made of an absorbing “black”    material which essentially absorbs 100% of the light received by it.    Its surface is typically a non-flat diffuser, so that light    reflected by it would be diffused and pose minimal risk. However,    using a beam block, of the type that is described in the above    referenced WO 2007/036937, involves a significant thickness increase    of the receiver as can be seen in FIG. 1 of that application. A beam    block would have to be block any reflected beams within the field of    view (FOV), which are typically reflected at a wider angular spread    compared to the FOV, but at the same time, not block any beam within    the field of view. Such a beam block height would have a very    significant impact on the overall receiver height.-   3. Diffusing the beam impinging on the receiver lens front    surface—however this solution is not suitable for use inside a laser    resonator, as it would distort the laser beam in a way that would    not allow the wave front to be recreated after a round trip.

In the light of all of the above described disadvantages, it is clearthat alternative solutions must be found in order to reduce potentiallydangerous reflections from the input lens of the receiver.

Furthermore, in such prior art receivers, a single photovoltaic cell isgenerally placed directly behind the back mirror output coupler of thesystem, at a suitable distance so that the beam is spread to thediameter where maximal efficiency of the photovoltaic cell is achieved.This also increases the overall depth of the receiver, which isdisadvantageous for such an application. It is clear that the prior artpositioning and configuration of the photovoltaic cell(s) has a numberof disadvantages, the trade-off between which is always a compromise toperformance.

There therefore exists a need for a thin receiver unit of a distributedlaser resonator which overcomes at least some of the disadvantages ofprior art systems and methods.

The disclosures of each of the publications mentioned in this sectionand in other sections of the specification, are hereby incorporated byreference, each in its entirety.

SUMMARY

The present disclosure describes new exemplary systems for the receiverunits of a distributed resonator laser power transmission system,utilizing novel configurations to overcome the above mentioneddisadvantages of the prior art systems. A combination of a robust frontoptical surface, such as a high abrasion resistant coating or a highabrasion resistant glass, a small beam block input beam size, andselected photovoltaic cell position has to be used in order toconstitute a safe thin receiver.

Some exemplary systems described in this disclosure overcome theseproblems, and attempt to fulfil at least some of the criteria mentionedhereinabove, by using a combination of the following techniques:

-   (a) A front surface, or a front surface coated with a coating, which    is durable enough to withstand normal wear and tear, but which is    optimized primarily for durability, and only then for AR, if at all.-   (b) Use of a beam blocker which collects the above-mentioned    reflected light from the front surface coating, or from the front    surface itself if uncoated; and-   (c) manipulation of the diameter of the transmitted beam impinging    on the receiver by use of a dynamic focusing system in the    transmitter unit for adjusting the position of the focus of the    transmitted beam. The focussing system may be passive or    self-adjusting.

Most anti-reflection coatings, especially those with very lowreflectivity of the order of 0.1% or better, cannot withstand typicaluse by a domestic user, such as normal scratching, or wear and tear.Furthermore, their reflection will increase when coated with a layer ofliquid, which may be very common in normal use of consumer products suchas cellphones. In order to reduce the effect of local conditions on thelevel of light reflected from the optical input element, the coating onthe front surface may be selected not to decrease the reflection to aminimum, but rather to reflect at least a certain level of the lightimpinging upon it, such a level of reflection generally having asignificantly lower dependence on the surface conditions, than aspecifically designed low reflection coating. However, such a level ofreflected light creates a safety risk unless properly blocked ordiffused and will also reduce system efficiency significantly.

To avoid the above problems, the optical systems of the currentdisclosure differ from conventional laser cavities in that they use thefront optical surface of the receiver as an output coupler instead ofhaving the output coupler at the end mirror, as is normal in prior artlaser systems. Prior art lasers generally position the output coupler asthe mirror at the remote end of the cavity. This configuration yieldsminimal losses, since the output coupled beam traverses the outputcoupler only once in each round trip, and this conventionalconfiguration also requires a minimal number of components, has minimalaberrations and most importantly, involves a single output coupled beam,emitted in a known position and of a known size towards a knowndirection, which can readily be directed onto a single photovoltaiccell. The positioning of the output coupler on the front optical surfaceof the receiver, as used in the systems of the present application, hasthe disadvantage that the output coupled light is extracted from twobeams coming from unknown positions on the front surface, in unknowndirections, and which are difficult to collect and to direct onto aphotovoltaic cell.

However, a first important advantage which this configuration providesis that the front surface does not need to be coated with ananti-reflective coating since the reflection therefrom is positivelyused to constitute the output coupled power (and would therefore beblocked and absorbed by (two) photovoltaic cells), and the front surfacecan thus be optimally coated with a coating which provides durabilityand scratch resistance, rather than minimal reflectivity.

A second advantage is that if a liquid is spilled on the front surface,or if the front surface is contaminated with dirt or fingerprints, or ifa flat reflector is inadvertently placed on the front surface, the lightreflected by the reflector or the front surface of the liquid will bereflected in essentially the same direction as the output coupled lightand therefore collected by the collection system instead of posing asafety risk. This configuration therefore allows all reflections fromthe front surface to be safely blocked, absorbed or diffused, and alsoenables efficiency to be high, even when this surface is coated with acoating reflecting more than just small fractions of a percent.

Additionally parts of the input optical surface can be made of a lightabsorbing material, such as a Photovoltaic cell (PV cell) which willfurther reduce the risk of stray light reflection. In addition, in someconfigurations where the entire front surface need not be of goodoptical quality, parts of the front surface may be adapted to diffusethe reflected beam for increased safety.

In order to mitigate the problem of the beam being reflected from anunknown position, the receiver may advantageously have an optical designsuch that its entrance pupil is located in the vicinity of the frontsurface, this minimizing both the size and complexity of the opticalcollection systems as well as the overall surface “footprint” of theaperture on the device, since the entrance window needed would besimilar to the beam size, and not to the beam size summed over theentire field of view.

If the front surface acts as an output coupler, the beam block blockingthe reflection from it needs to match the front surface and becomes a“smart beam block”, with additional capabilities beyond just absorbingthe light, which is what a conventional beam block does. Firstly, itneeds to be able to collect the light coming from the input pupil of thereceiver from all directions, and to use it by converting it into auseful form of energy, such as directed and localized optical power,heat, or electrical power. If electrical power is needed then typicallythe size and shape of the beam would have to be optimized for aphotovoltaic cell, which typically performs best with a flat beamprofile and not the Gaussian-type beam profile generated by the system.To convert the beam profile to a flat shape, a hologram, a lensletarray, a shaping prism, a light-guide beam scrambler, or a diffuser maybe used.

Additionally, the “smart beam block” may also perform other functions,such as monitoring the incoming light in order to detect other riskssuch as contaminations on the surface of said receiver, or to detectdata transmission “riding” on the beam.

In order to successfully perform the above functions, the front surfaceof the beam block should typically be transparent or semitransparent,allowing the light to enter the beam block, where additional optical andelectro-optic components may perform functions on said light, before itis absorbed as ultimately intended. Alternatively the front surface maybe made out of an electro-optical active material, such as asemiconductor, that may directly convert the light energy intoelectricity, as a more useful form of energy. In such a case, the frontprotective cover of the photo-voltaic cell may be considered to be theinput to the reflected beam blocker. The semiconductor may also be“segmented”, with each segment providing information about portions ofthe beam, so that data on the beam shape, and possible contaminations onthe front surface may be gathered.

Many such “smart functions” may also be performed by a “smart” outputcoupler. Thus, detectors for detecting beam shape and surfacecontaminations may be embedded in or just behind the front surface.Markers for detection and identification of the receiver by thetransmitter, such as a bar-code or other optical code with identityinformation identifying the receiver, may also be embedded within thefront surface. Energy converters, heat removal units, retro reflectors,diffusers and signaling systems (such as LED diodes optically signalingto the transmitter) may also be embedded in the front surface. Such subsystems in the front surface should be built in such a way so that theydo not reduce the quality and quantity of the returning signal below thepoint allowing the transmission system to continue to operate, hencethose should preferably be embedded in the outer part of the frontsurface, and further preferably be embedded behind a protective frontcover which is made to be durable.

The optical focusing system of the receiver can be either lens based ormirror based. If mirror based, the input and focusing unit can beconveniently implemented in the form of a solid prismatic block ofoptical material such as glass, with the input surface being optionallyplanar, and with the focusing mirror formed as a concave, preferablyconic, surface on which the beam impinges and is focused onto a secondmirror operative as the retro reflector.

Whether a lens or a mirror focusing element is used at the optical inputof the receiver, there will typically be two reflections formed at theinput window—one from the transmitted beam incident externally on thereceiver and one from the beam returning from the retroreflector,incident internally on the input window, and passing through the inputwindow in the other direction, outwards towards the transmitter. Sincethe receiver optical system has an output coupling and loss ratio whichis determined by its design, the ratio of power between the two beamswould be approximately constant As an example to illustrate this, if theincoming beam is 10 W (optical) and the front surface output coupler,being uncoated, reflects 4% by Fresnel reflection, then the output ofthe first beam would be 400 mW, and the returning beam would be 0.04*9.6W=384 mW, and even if the incoming power fluctuates, the ratio of outputcoupling 1 and output coupling 2 would remain constant. Therefore,unless a system could be designed with output coupling exceeding 50%,which is not normally possible using gain media suitable for distributedresonators, it is important to utilize both beams in extracting energyfrom the cavity, and the optical beam block geometry must be arrangedsuch that both beams are collected, and the output from both collectionsystems typically need to be summed.

The collected light from each of the reflections may conveniently becoupled into its optical waveguide or optical fiber, and the opticalpower in both of these waveguides or fibers may be combined and useddirectly. In the case of electrical outputs, separate photovoltaic cellsmay be used at the ends of the waveguides or optical fibers, and theiroutputs typically summed. There will be a typically small, typicallyfixed, difference in the power received by the two photovoltaic cells,since the externally reflected light has undergone only one reflectionon the input surface, while the internally reflected light has undergonetwo reflections on the input surface. The difference is only of the sameorder as the percent reflection of the light from the input surface.However, in order to maintain balanced operation and conversion withmaximum efficiency, a current matching device should be used. Such apower matching device could be a DC/DC converter on at least one of thephotovoltaic cells or an optical assembly diverting more power to theinternal reflection cell, in order to compensate for its lower inputpower. Alternatively, a third cell may be used which may receive some ofthe power impinging on one of the first two cells, so that the first twocells, or preferably all 3 cells are “balanced” and can be efficientlycombined. For example, in the simplistic case of 50% output coupling,the beam coming from the transmitter to the receiver would couple 50% ofits power into a first channel, and 25% (50% of 50%) of its power againreturning from the receiver to the transmitter. Since the first channelis exactly two times the second channel, if power from the first channelis evenly split between two photovoltaic cells then all three PV cellswill receive exactly the same power.

The thickness of modern cellular telephones and tablets is generallymade as small as possible, because of consumer preference andrequirement. In conventional laser design, the number of surfaces insidethe laser resonator is reduced as much as possible, since eachadditional surface creates an additional loss in both the forward andbackward propagation direction of the laser light, and those losses arein competition with the output coupling and thus may reduce efficiencysignificantly. In contrast to such conventional designs, the opticalreceivers of the present disclosure may use folding mirrors, in order tofold the retroreflector and collection systems, such that they lie inthe plane parallel to the front surface of the device, in order tominimize the thickness of the optical receivers while still keeping thefield of view centered orthogonal to the surface of the cellulartelephone such that it can be charged from a transmitter placed on theceiling when the telephone is placed lying on a table.

Other functions that could be performed by the “smart beam block” or“smart front surface” include manipulating the beam shape, typically toeither improve photovoltaic cell efficiency by homogenizing the beam, orin order to bring light coming from different directions to a specificsmall point in space, typically in order to couple it into a waveguide,or diffusing the light typically for safety or illumination purposes

Other functions may include converting the light to one or morewavelengths, for example, creating RGB light from the lasing light,typically by upconverting the photons for wireless illuminationpurposes. This may be done by using Yb doped glasses, or othertransparent materials doped with fluorescent materials and absorbingmaterials such that energy is upconverted and transformed. For example,laser light can be absorbed by a transparent glass doped with Yb, whichhas a very wide absorption around 1000 nm. Such a dopant may absorb thelight and transfer its energy to other co-dopants that may absorb energyand eventually emit their energy as fluorescent light. The pupil of thereceivers described in the prior art, such as in the above mentioned WO2012/172541, is typically located at the center of the lens used, or, inthe case of a telecentric optical system, one focal length in front ofthe input optical aperture plane, or a similar distance within thereceiver. Placing the pupil near the front surface reduces the field ofview and significantly limits the possible optical designs that could beused. Furthermore, a requirement that the front surface be flat, ornearly flat, typically causes aberrations that reduce systemperformance. However, in the optical systems of the current disclosure,in contrast to prior art system design, the pupil is arranged to fallclose to the front surface, and the front surface is made flat, ornearly flat. Making the front surface flat, or nearly flat, anddesigning the pupil to be close to it, substantially minimizes the sizeand complexity of the smart beam block collection systems, since all ofthe beams handled over a range of incidence angles, pass through thepupil of the system, thereby significantly simplifying the opticaldesign. Such a design, where the entrance pupil is placed on the frontsurface of the receiver, also minimizes the area of the input windowneeded on the front surface of the device, surface area which may becrucial in many such mobile applications such as cellular phone designnot only from a marketing point of view but also as it is proportionalto the overall thickness of the device.

The output coupling level in prior art conventional laser systems istypically adjusted to provide optimal performance of the gain medium,this resulting in maximal lasing efficiency. Using this prior artconventional criterion, the effect of dust/finger print and othersurface losses would be taken into account and the final design, basedon conventional wisdom, would require the receiver system to couple LESSlight out and reflect MORE light back to the transmitter compared to thelevels that would have been selected to optimize gain medium utilizationin the absence of such losses. Such a system will thus continue tooperate even if the front surface of the receiver is soiled withfingerprints or dirt arising from to day-to-day consumer use.

The optimum output coupling factor for such a conventional laser designis well known in the art, as given in a number of textbooks, especiallyas derived in the classic work by A. E. Siegman entitled “Lasers”published by University Science Books, (1986), where there is stated onpage 479 under the heading “Optimum Output Coupling Factor” (with someof the mathematical details omitted for simplicity):

“For any of the lasers shown in FIG. 12.11 or 12.12 (not shown herein)there is obviously a maximum allowable output coupling, . . . beyondwhich the cavity is overloaded, so that total cavity losses exceed theavailable gain, and no oscillation is possible. As the cavity couplingor end-mirror transmission is reduced below this value, both thecirculating intensity and the output intensity increase with decreasingcoupling. Below a certain optimum coupling factor . . . , however, themirror transmission decreases faster than Icirc increases, and the poweroutput decreases, eventually becoming zero at zero transmission throughthe end mirrors. The laser at this point is, of course, stilloscillating—in fact, oscillating the strongest of all—but with all itsavailable power being uselessly dissipated in the internal cavitylosses.

FIG. 12.15 (not shown herein) illustrates in more detail how the laseroutput intensity for a typical laser depends on the cavity outputcoupling, assuming a fixed value of 20% power gain per round trip, andvarying amounts of internal cavity loss. It is evident that for eachdifferent value of internal cavity loss there is a different optimumoutput coupling which maximizes the output power. It is also apparentthat the optimum output coupling is always considerably smaller than theavailable gain, and that even very small internal losses have a veryserious effect on the maximum useful output power available from thelaser.”

However, contrary to this conventional wisdom, as expressed by Siegman'soptimal recommendations, the systems described in this disclosuregenerally use total output coupling—resulting from the sum of the outputcoupling of both beams—greater than that determined for the optimum ofthe gain medium. This unconventional design step is taken purposely inorder to provide two advantages:

-   (a) Such a system is more tolerant to the effect of finger prints    and other “parasitic” losses on the output coupler. The term    “parasitic” losses is applied here to losses that are not due to    components inserted into the laser by the laser designer but rather    are introduced into it by the use of the system. Such parasitic    losses are negligible in conventional laser design but are critical    in the context of the current described systems.-   (b) Furthermore, such a system will be safer, as lasing would    collapse if a reflector were to be inadvertently placed on the front    surface. Contrary to conventional laser systems, where such a    collapse would be considered a malfunction, in a system such as    those shown in the current disclosure, it is a required feature.

As an example, if the gain medium of a certain laser works optimallywith 90% of the light reflected back to it, conventional wisdom woulddictate coupling <10% of the light out of the resonator while reflectingmore than 90% of the light back towards the transmitter, so that when anadditional loss is inserted into the system, the system would continueto operate. The above mentioned Siegman reference, as well as almost anylaser textbook, give a formula for this number. However the cavitydesigns shown in the present disclosure may chose to couple out morethan 10% of the light, and reflect back less than 90% of the light, sothat although the system would be less efficient, it would benefit froman additional safety factor thanks to two factors:

-   (a) The circulating power inside the resonator would be lower.-   (b) The system's “self terminating” safety effect would become    stronger.

In order to construct an optical receiver that would fit into a thinportable electronic device, such as a cellphone or a tablet computer,the optical thickness of the retroreflector and the height of the beamblock needs to be minimized. Since both of these heights areproportional to the size of the pupil, the optical receiver systems ofthe current disclosure attempts to minimize the diameter, or overallsize, of the entrance pupil, at the cost of complicating the transmitterand receiver systems. It is important that the pupil diameter should notbe too small, since producing a beam of diameter below approximately 0.3mm radius, requires the use of very large optics in the transmitter tosupport the numerical aperture needed. This in turn would make thetransmitter large, expensive and slow to focus. It is also important notto allow the intra-cavity optical intensity (W/cm²) to become more thenthe damage threshold of the receiver optical components or more than thethreshold at which dust in the air ignites. For this reason, an optimalfocusing lens for the transmitter would generally have a numericalaperture of less than 0.01, and optimally around 0.001.

There is thus provided in accordance with an exemplary implementation ofthe devices described in this disclosure, a receiver for receiving anincident beam of optical power from a remote transmitter over apredefined field of view, comprising

-   (i) an optical input element positioned such that it can receive the    beam of optical power over the field of view,-   (ii) a retroreflector disposed such that it reflects back towards    the optical input element, that part of the incident beam of optical    power traversing the optical input element,-   (iii) at least one reflection blocking element having a transparent    optical impingement surface, disposed such that reflections of part    of the incident beam off the external surface of the optical input    element, impinge on the transparent optical impingement surface, and-   (iv) at least one photovoltaic cell disposed behind the transparent    optical impingement surface, to convert light from the reflections    of part of the incident beam into electricity.

In such a receiver, the transparent optical impingement surface may bethe entrance window of the at least one photovoltaic cell. Additionally,it may be the entrance window of a light guide, having the at least onephotovoltaic cell disposed at an end of the light guide remote from thetransparent optical impingement surface.

In any of the above described receivers, the optical input elementshould have an external optical surface having a mechanical durabilityselected to withstand a predetermined level of handling. This externaloptical surface may comprise either a glass produced to have a highresistance to mechanical damage or a glass having a scratch resistantcoating thereupon. In either of such cases, the predetermined level ofhandling should be determined according to the expected effects ofdomestic use of the receiver.

Additionally, such receivers may further comprise at least a secondphotovoltaic cell disposed internally from the optical input element,such that it receives reflected light from the optical input elementarising from that part of the incident beam of optical powerretroreflected back towards the optical input element. In thatsituation, the receiver may further comprise a light guide for conveyingthe reflected light to the at least second photovoltaic cell.

Still other example implementations involve a receiver for receiving anincident beam of optical power from a remote transmitter over apredefined field of view, the receiver comprising:

-   (i) a lens having a front surface,-   (ii) a retro reflector disposed within the receiver such that it    retroreflects parts of said incident beam traversing the lens back    to the lens, and-   (iii) at least one beam block, disposed such that it intercepts    reflections of part of the incident beam off the front surface of    the lens, the beam block being equipped with an energy conversion    device,-   wherein the front surface of the lens has a reflectivity of at least    0.01%, and has either a high durability coating or is constructed of    a scratch resistant glass selected to withstand a predetermined    level of handling. In such a receiver, the energy conversion device    may comprise a photovoltaic cell. Such a receiver may further    comprise a beam homogenizer disposed between the beam block front    surface and the photovoltaic cell.

In the above described receivers, the retro reflector may have a pupillocated close to the front surface of the lens. The location of thispupil close to the front surface may be achieved by disposing theretroreflector substantially at the focal plane of the lens.

Furthermore, in the above described receivers, the coating may have apredetermined level of resistance to at least one of scratches, spillsand fingerprints.

According to yet further implementations, the energy conversion devicemay be a heat generator.

Finally, the receivers may further comprise an additional energyconversion device disposed such that it intercepts internal reflectionsof part of the incident beam off the front surface of the lens, arisingfrom parts of the beam returning to the optical input element from theretroreflector.

Yet another implementation of the receivers of the present disclosuremay further involve a receiver for optical power transmission, thereceiver comprising:

-   (i) an input optical receiver unit comprising a block of optically    transparent material, the block comprising an input surface, a    concave focusing mirror and a retroreflector mirror disposed at the    focus of the concave focussing mirror, and-   (ii) at least one beam block, disposed such that it collects any    light reflected from the input surface, the beam block being    equipped with an energy conversion device,-   wherein the input surface has a reflectivity of at least 0.01%, and    has a coating having a durability such that the input surface can    withstand a predetermined level of domestic wear and tear.

Finally, according to yet another exemplary implementation of thedevices of the present disclosure, there is provided a receiver foroptical power transmission, the receiver comprising:

-   (i) a lens having a front surface, the lens being disposed in an    optical entrance aperture of the optical receiver,-   (ii) a mirror disposed inwards of the lens relative to the incident    direction of the optical power transmission, such that the lens and    the mirror define a retro-reflector, the lens being disposed such    that the entrance pupil of the optical receiver lies at the front    surface of the lens, and-   (iii) at least one photovoltaic cell disposed in the optical    aperture in the plane of the lens,-   wherein the photovoltaic cell has an area substantially larger than    that of the front surface of the lens, such that a majority of the    optical power incident on the entrance aperture is absorbed by the    photovoltaic cell.

Such an optical receiver may further comprise at least one beam block,blocking essentially all reflections from the front surface of the lens,of the optical power coming from the field of view. Such a beam blockmay comprise a second photo voltaic cell for absorbing the reflectionsof beams coming from the field of view, and this second photovoltaiccell may be disposed surrounding the lens.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawings in which:

FIG. 1 shows a prior art receiver using an optical absorber baffle toabsorb light reflected from the front surface of its input lens;

FIG. 2 is a schematic representation of a distributed resonator lasingsystem, according to one exemplary implementation of the systems of thepresent disclosure;

FIG. 3 is a detailed schematic illustration of one exemplaryimplementation of the receiver configuration shown in FIG. 2, using aninput lens focusing element; and

FIG. 4 is a detailed schematic illustration of another exemplaryimplementation of the receiver configuration shown in FIG. 2, using areflective focusing element,

FIG. 5A is a schematic illustration of the general use of a smallcentrally located retro-reflecting region in the center of aphotovoltaic output coupler; FIG. 5B illustrates a specific geometricalarrangement, in which the entrance pupil of the optical input system islocated on the surface of the lens, while FIG. 5C illustrates the casewith the entrance pupil above the surface, and FIG. 5D is a plan view ofan alternative construction of the optical input aperture in which anumber of retro-reflecting elements are arranged around the center ofthe input aperture, while the rest of the area is taken up by thephotovoltaic cell output coupler arrangement; and

FIGS. 6A, 6B and 6C show possible configurations of the locations of theentrance aperture and the beam blocker on exemplary mobile phones.

DETAILED DESCRIPTION

Reference is now made to FIG. 1, which illustrates schematically theinput lens 10 of a prior art receiver using an optical absorber baffle11 in order to absorb light reflected from the front surface of thelens. The baffle 11 is located in its predisposed lowest position shownin FIG. 1, so that the outermost limits 12, 13 of the fields of view ofincident beams, as limited respectively by any input aperture (notshown) and by the edge of the baffle 11, result in beams 14, 15reflected from the front surface of the lens, and stopped by the baffle.However, as is observed in FIG. 1, in order to prevent any of the beamswithin the field of view 12, 13, from being reflected back towards theenvironment around the receiver, even this lowest position of the baffleis noticeably distant from the front surface of the lens, therebyincreasing the effective height of the optical receiving system. Thisprior art solution is therefore generally impractical for use on thinconsumer electronics devices.

Reference is now made to FIG. 2, which is a schematic representation ofa distributed resonator lasing system, according to one exemplaryimplementation of the systems of the present disclosure. The system hasa number of features which differ from those of the prior art systemsshown hereinabove.

In the first place, the transmitter unit 25, besides the gain medium 27and the retroreflector 28, there is a dynamic self-adjusting focusingelement 20 in order to ensure that the focal point is generated on thelens 21 of the receiver. Dynamic focussing may be defined as relating toa system where the diameter, wave-front radius of curvature and M² valueof the beam leaving the transmitter is dependent on the distance fromthe transmitter to the receiver. This dynamic focusing element ensuresthat the beam impinging on the receiver has three characteristics:

-   (i) it is not significantly larger than the front surface optical    entrance aperture of the receiver, which may be semi-transparent    either by means of partial reflection from the whole area, or by    means of full retro-reflection from certain portions of the area,    usually the center of the aperture, and output coupling from other    areas. Typically, for use in a mobile hand-held device, such as a    cellular telephone, the size of the entrance pupil should be smaller    than about 2 cm to fit comfortably on the front surface of a    cellular phone.-   (ii) it does not yield power densities greater than a threshold    level which could damage the optical surfaces of the input lens    (i.e. it does not focus too much); and-   (iii) it maintains the desired focused diameter at the receiver, by    either optical, mechanical, electronic, active or passive means,    despite the changing distance from the transmitter to the receiver    as the position of the receiver is changed.

The input lens 21 inside the receiver may advantageously have a frontsurface 23 which is essentially flat, and a back surface which istypically an aspheric surface, as will be shown in FIG. 3. Additionally,the front surface 23, since it is intentionally coated with a durablerather than an antireflective coating, reflects a part 22 of theincoming beam. In order to resist everyday wear and tear, it should beconstructed of a scratch resistant glass, such as Gorilla Glass®,supplied by Corning Incorporated of Corning, N.Y., USA, which may alsobe treated with a scratch resistant coating. A reflection of the orderof 3% to 6% is typically obtained. Other suitable high durability frontsurfaces include Sapphire coatings, Diamond-like coatings, Conturan®Darocoatings as supplied by SCHOTT North America Inc. of Elmsford, N.Y., andother coatings and glass types.

The beam blocker 24 which may be disposed opposite the front surface ofthe lens to intercept light reflected from the lens, has a novelconfiguration in that it is configured to be essentially transparent tocollect the light incident on it, rather than opaque and absorbent, asin conventional prior art beam blockers. As will be shown in thedetailed drawings of FIGS. 3 to 5 below, the light 28 transmitted pastthe beam blocker front surface 24 is detected by a photo-detector 29,such as a photo-voltaic cell. The photo-detector can be disposed eitherimmediately behind the beam blocker front surface, as shown in FIG. 2,or the beam blocker may even be the front surface cover of thephotodetector, or the photodetector can be at the remote end of a lightguide which can also function as a beam scrambler, or it can surroundthe entrance pupil. A fuller explanation of the use of a photovoltaicdetector surrounding the entrance pupil is given in connection with FIG.5 hereinbelow. The beam blocker 24 with its associated photovoltaic cell29 can thus be used as an output coupler system, using the reflectionfrom the front surface 23 of the input lens 21 as the light coupled outof the resonator. This novel system therefore, utilizes theintentionally non-suppressed or under-suppressed reflection of lightfrom the front surface of the input lens in order to perform at leastpart of the power extraction from the laser resonator. The lightcollection system, such as a light guide, may also be configured toperform at least some of the functions of collecting the light impingingon the beam blocker, converting it into a flat profile beam,manipulating its size and directing it towards the photovoltaic cell.

The system shown in FIG. 2 is a safe system because the distributedresonator design of the system ensures that lasing light only impingeson the input lens from directions within a predefined field of view, asthose directions are the only ones where clear line of sight existsbetween both retro reflective members of the resonator. Lasing can thusonly occur from a predefined solid angle when looking from the receiverside, and it is therefore possible to position and size the beam blockerso that it blocks any reflections from the front surface originatingfrom beams coming from within that field of view, thus ensuring thatonly beams that will be blocked by the “smart” beam block could betransmitted to the receiver.

Reference is now made to FIG. 3, which is a schematic illustration ofone exemplary implementation of such a receiver configuration. Thereceiver should be configured such that its entrance pupil is at thefront surface of the input optical window, which could be the frontsurface of the focusing lens. The lasing beam 30 to and from thetransmitter enters the front surface 23 of the lens 21, and is focusedthereby towards the back mirror 31, the whole of this structure actingas a retroreflector. A folding mirror 32 may be conveniently disposed topreserve the thin geometry of the receiver configuration. The backmirror 31, disposed at the image plane of the input lens 21, passes thelasing beam back through the lens 21 and towards the transmitter unit.As the focusing element has a numerical aperture generally smaller than0.01, focusing is rather weak. However it should be appreciated thattypically the beam is 2-3 times wider at the transmitter compared to thereceiver, and for a typically sized transmission distance of a fewmeters, the size of the beam 30 on the input surface 23 of the receiveroptics 23, may be of the order of a millimeter or so.

Since, as described above, the front surface 23 of the lens may notnecessarily be anti-reflection coated, a percentage of the incoming beamis reflected therefrom, and into the front surface 34 of the beamreflection blocker. The photo-voltaic cell to extract this outputcoupled light may be situated immediately behind this front surface 34,but this location may be inconvenient for the aesthetic and size designconsiderations of a mobile device. Therefore, in the implementationshown in FIG. 3, the photo-voltaic cell 36 is situated at the end of afirst light pipe 33. This allows for more flexible location of thephoto-voltaic cell in the mobile device. Since the light pipe 33collects light reflected externally from the front surface 23 of thelens, this light pipe is known as the external reflection light pipe.The collected light may then be directed by means of a high reflectivitysurface 35 down the length of the light pipe, towards a firstphotodetector 36 situated at the end of the light collector. The sidewalls 37 of the light pipe, except the input window and the foldingmirror surface, may be textured in order to scramble the light beam inorder to convert the input laser beam, which may have a profile close toa Gaussian mode, to a top-hat profile in order to produce a more uniformincident flux on the photovoltaic detector. This beam profile willincrease the conversion efficiency of the photovoltaic cell, since theentire surface can then be used at its optimum efficiency withoutneeding to limit the photocell characteristics to match the beamintensity over a small central area of its surface where the beamintensity may be substantially higher than in the peripheral areas ofthe detector. The smaller the cross section of the light guide 33, thelarger the number of reflections the traversing light makes off its sidewalls, and the more homogeneous the beam incident on the photovoltaiccell 36. To maintain minimum height from the receiver surface, the lightguide should be positioned as close as possible to the lens, but withoutblocking the desired field of view. A lower profile beam blockingarrangement than that of the prior art beam blockers is achieved becausethe light guide/baffle/collector is placed as close as possible to thefront surface. Since the front surface has a minimal size, the lightpipe height is also minimal.

In addition to the external front surface output coupler arrangement,this novel type of system may also have a second collection system tocollect the light reflected from the inner surface of the input lens 21,incident thereupon from the retroreflector 31. This generates additionaloutput coupling from inside the receiver unit. This internal outputcoupler arrangement may include a second light pipe 38, known as theinternal reflection light pipe. It may have the same features as theexternal light pipe, and it is terminated in its own photodetector 39.The output coupling is thus performed by adding the power collected atboth the photodetectors 36, 39.

The reflectivity of the front surface 23 of the lens 21 should beselected so that the output coupling is configured to be somewhat morethan the optimal level for the gain medium taking into account dust,fingerprints and other contaminations on the surface, in order toprovide increased safety. Since the transmitter in the distributedlasing systems of the type described in this application generally use athin disk gain media, which has low saturated gain—typically less than 1dB—a total output coupling, including power losses, of more than 10 to15% will typically terminate lasing, or at least reduce the powersignificantly.

Although the above described beam reflection blockers may transmit mostof the reflected energy to the photovoltaic energy collection system,for use in powering the device in which the receiver is installed, itmay also transmit some of the energy to a monitoring system, which canbe used, for instance, for monitoring the status of the input surface ofthe focusing element, such as due to dirt, fingerprints, and the like,or for detecting transmissions such as data transmissions from thetransmitter. Furthermore, the front surfaces of the reflection beamblockers 34 may be diffusing surfaces, such as to assist in thescrambling of the detected beam.

Reference is now made to FIG. 4, which schematically illustrates analternative receiver configuration, in which a reflective mirror surface40 is used as the focusing element instead of the lens of theimplementation of FIG. 3. The implementation shown in FIG. 4 isespecially convenient in that the optical assembly can be constructed ofa single piece of transparent materials such as glass, with opticalfibers attached thereto for conveying the output coupled light to thephoto voltaic detectors. An input beam within the field of view, FOV, isincident on the input entrance surface, which is a plane transparentwindow 42. The optical properties of the window should be adapted towithstand the handling environment of the mobile device such as acellular telephone in which it is installed, and may be advantageouslyconstructed of Gorilla Glass®, or a scratch resistant coating on theouter surface of the focusing block. A plane mirror surface 43 inconjunction with a parabolic focusing mirror surface 40, generates theretro-reflected beam. The input pupil 41, is located just inside of theentrance window 42.

The light of the input beam reflected from the front surface of theinput window 42, which is typically in the region of 3% to 15%,converges on the external image of the pupil 41 e, and is directed intothe external reflection light guide 45, which can conveniently be anoptical fiber, which conveys the illumination to an externalphotovoltaic cell 46. The light from the retro-reflected beam, reflectedfrom the inner surface of the input window surface 42, converges on theinternal image of the pupil 41 i, and is directed into the internalreflection light guide 47 where it is conveyed to the internalphotovoltaic cell 48. Although the light guides are schematically shownin FIG. 4 as fibers, it is to be understood that they can be configuredas solid light guides to fit the space limitations of a cellular phone,for instance, by means of a designed shape, the use of at least onehighly reflecting surface, and textured sidewalls, in the same way aswas described for the light guides of FIG. 3.

In all of the above described configurations, single photovoltaic cellsare preferably used at each photovoltaic location, rather than multiplephotovoltaic cells, since efficiency drops significantly using multiplecells for the following reasons.

-   1. Light falling on “gaps” between the cells is not converted to    electrical power, and is lost.-   2. Such multiple cells would need to be connected either in series    or in parallel. If connected in series, the current flowing would be    limited to the current generated by the cell producing the least    amount of current, to the limit that if one cell is not illuminated    at all, the system would not generate any power. Thus, any    inhomogeneity in the beam would reduce system efficiency.

3. On the other hand, series connection would have the disadvantage thatthe voltage generated by the cells would essentially be equal to thatproduced by a single cell, with added loss attributed to the power lostto gaps and to the added complexity.

In cases where multiple photovoltaic cells must be used, such as if alarge area has to be covered, then such cells must be connected inparallel and packed as closely as possible.

The above described receiver examples incorporate two separate outputcoupling systems, each with its own photovoltaic cell. The ratio oflight falling on the two cells is fixed, and is determined by theoptical properties of the front surface coating. The cells can thereforebe connected in series, each with its own DC/DC converter to match thecurrents of both cells.

In order to ensure that a lasing beam impinges on the receiver inputwhile fulfilling the criteria described in connection with FIG. 2, thetransmitter is equipped with a dynamic focusing system. The simplest wayof achieving this is by obtaining a measure of the spot size on thereceiver input, and providing feedback to the transmitter to maintainthe spot size at its predetermined level. There are several possibleways of achieving this, such as:

-   (a) Measurement of the beam size at the transmitter. This is    directly proportional to the focused beam size at the receiver,    since the resonator is stable.-   (b) Measurement of the power detected in the receiver, and    transmission of a signal to the transmitter which is used to    maximize that detected power.-   (c) Measurement of the power circulating within the resonator, i.e.    between the transmitter and receiver, since this is proportional to    the output coupled power. This power is most conveniently measured    at the transmitter output.    Any form of autofocus may be used in the transmitter, actuated by    the feedback signal generated by one of the above three methods, or    an alternative suitable method.

Reference is now made to FIG. 5A which illustrates schematically animplementation in which a retroreflector is used, operative over only asmall part of the area of the laser beam incident on the input aperture,most conveniently over the center area, with a photovoltaic cellcovering the rest of the area, such that the output coupling ratio willbe very high, possibly up to 85 or 90% or even more. The photovoltaiccell 50 covers most of the area of the optical input aperture of thedevice, and at the center of the photovoltaic cell, there is situatedthe input lens 51 with a back mirror 52 disposed at its focal lengthbehind it. Since the input pupil of the receiver optical system issituated at the front surface of the lens, the lens/back mirrorcombination constitutes the retroreflector of the lasing systemreceiver. The retroreflector thus covers only a small part of the inputbeam 54, and since the power density of the input beam is highest at itscenter, in order to retroreflect a desired percentage of the total powerof the input beam, which could be of the order of 10 to 40% of the beam,the area of the retroreflector can be even smaller than that expectedfrom a simple geometric ratio based on percentage of power to bereflected. The retro-reflected beam 55, then diverges back towards thetransmitter, and if the gain medium has sufficient gain, it is againamplified and returned by the retroreflector in the transmitter to theinput aperture of the receiver, where again the major part of theincident beam is absorbed by the photvoltaic cell and the smaller partis reflected back towards the transmitter, and so on. The large areaphotovoltaic cell thus acts as a high ratio output coupler for theincident beam from the transmitter, extracting power from the cavity forcharging the mobile device.

For such a system to operate, the entrance pupil must reside very closeto the surface of the beam block, whether photovoltaic sensors, adiffuser or a front window. If the pupil resides below or above thesurface then the area of the focusing element 51 on the surface wouldhave to increase to cover beams coming from different directions. Insuch a case, the excess power falling outside of the focussing element51, that should be blocked by the beam block, would be significantlysmaller.

This can be illustrated in FIGS. 5B and 5C, where FIG. 5B illustratesthe case with the pupil on the surface, such that the aperture equalsthat portion of the beam to be reflected, while FIG. 5C illustrates thecase with the pupil above (though a similar effect would be obtainedwith the pupil below) the surface, in which case the aperture size isbigger than the designed beam.

The implementation of FIG. 5A is only one method by which aretro-reflecting element having a substantially smaller area than theoutput coupling element can be constructed, in order to achieve a highoutput coupling ratio. Reference is now made to FIG. 5D which is a planview of an alternative construction of the optical input aperture 56 ofa receiver of the present disclosure, in which a number ofretro-reflecting elements 57 are arranged in a radial pattern around thecenter of the input aperture, while the rest of the area is taken up bya photovoltaic cell arrangement 58 as the output coupler. Each of theretroreflector elements can be directed at a different region of thefield of view to increase the coverage of the field of view of thesystem. The pattern shown in FIG. 5D is meant to illustrate an exemplarypattern of retroreflector in a matrix of photovoltaic cells, and it isto be understood that any other arrangement can equally well be used,while maintaining the

It should be noted that the implementations of FIGS. 5A and 5D, in whichthe position of the photovoltaic cell arrangements is shown to be in thesame plane as the retroreflector, is not intended to be a definitiveposition for these beam output couplers. The novelty of theimplementations shown in FIGS. 5A and 5D is intended to be in the outputcoupling ratio used, which is substantially higher than is generallyused in the prior art. The photovoltaic cells could equally well bebehind a transparent window and at a remote position in the receiver, atthe end of a light pipe or waveguide, as shown in FIGS. 3 and 4. Theimportant feature of the implementations of FIGS. 5A and 5D lies in theratio of the area of the retro-reflecting surface or surfaces, to thatof the output coupling surface or surfaces.

One additional advantage of the type of implementations of FIGS. 5A and5D is that they are inherently safer than the previous implementations,and in most cases, where the lasing takes place outside of the visiblerange, such as at 1350 nm, do not even need a baffle in order to absorbany unwanted light reflected from the input aperture lens. This arisesbecause of the very small area of retro-reflecting surface to which thebeam is exposed. In the implementations of FIGS. 5A and 5D, thephotovoltaic cells are highly absorbing and therefore do not reflect anysignificant level of the incident level beam. Therefore essentially theonly stray reflected light comes from the front surface of the inputlens, which has a very small area. If the area is only 10% of the totalinput aperture, then 4% of reflected light from a typical glass frontsurface of the lens represents only 0.4% of the transmitted lasing beam,which may be well below the maximum permissible exposure at the nearinfrared wavelengths used. In contrast to this if a low output couplingratio is used, the area from which light can be reflected may be closeto 100% of the aperture, and 4% reflected stray light may generally beunacceptable.

Reference is now made to FIGS. 6A and 6B which show possibleconfigurations of installing the entrance aperture and the beam blockerof a receiver for a distributed resonator embedded in an exemplarycellular telephone. Since the beam blocker generally masks one lateralside of the beam transmission, it is necessary to provide at least twoinput apertures facing opposite directions, each with its own beamblocker in order to provide all-around coverage to receive thetransmitted beam. In the sample phone 60 shown in FIG. 6A, the opticalinput apertures of the lasing receivers are located in a steppedstructure 61 at one end of the phone. One input aperture 62 is locatedin a step perpendicular to the face of the phone with its blocker 63located opposite it on a step parallel to the face of the phone, whilethe other input aperture 64 is located on the step parallel to the faceof the phone with its blocker 65 on the step perpendicular to the faceof the phone. By this means, the field of view of the receiver extendsfrom the plane parallel to the face of the phone to that perpendicularto the face of the phone.

In the sample phone 66 shown in FIG. 6B, the optical input apertures ofthe lasing receivers are located in a V-shaped groove 67 across thewidth of the phone. The first input aperture 68 and its associatedblocker 69 are located on opposite sides of the groove at one positionacross the width of the phone, while the other input aperture 17 and itsassociated blocker 71 are also located on opposite sides of the groove,at a different position across the width of the phone, but facing thereverse direction to those of the first input aperture 68 and itsassociated blocker 69. In this configuration also, depending on theincluded angle of the groove, the field of view of the receiver extendsover approximately 90° centered on the perpendicular to the face of thephone.

In the sample phone 74 shown in FIG. 6C, the phone has a very smallretro reflector 75. The front surface window 76 may include a diffuserand scratch resistant coating as well as possible detection mechanismsfor contamination. A photovoltaic cell 77 is shielded from the user anda detector 78 used to further detect reflective contaminations on thesurface is also shown. Retro reflector 75 returns the central portion ofthe the incoming beam, creating a feedback signal to allow lasing tocommence. The front surface 76 absorbs the rest of the beam and convertsit to electrical power. The reminding photons that are not absorbed ortransmitted to an absorber behind front surface 76, are reflected byfront surface 76 and diffused by it. Radiation impinging on frontsurface 76 is mostly absorbed, but the remaining photons reflected by itare diffused so that they cannot form a safety hazard.

It is appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. Rather the scope of the present inventionincludes both combinations and subcombinations of various featuresdescribed hereinabove as well as variations and modifications theretowhich would occur to a person of skill in the art upon reading the abovedescription and which are not in the prior art.

1. A receiver for receiving an incident beam of optical power from aremote transmitter over a predefined field of view, said receivercomprising: an optical input element having an external optical surfaceselected to withstand a predetermined level of handling; at least onephotovoltaic cell disposed within said receiver, inwardly to saidoptical input element, to convert light from said incident beam intoelectricity; and a retroreflector disposed inwardly to said externaloptical surface, and covering a minor part of the area of the inputaperture, such that it reflects only a minor portion of said incidentbeam of optical power, said reflected incident beam thereby divergingback towards said transmitter, therein creating a feedback signal toallow lasing to commence.
 2. The receiver according to claim 1, whereinsaid external optical surface selected to withstand a predeterminedlevel of handling has a predetermined mechanical durability.
 3. Thereceiver according to claim 1, wherein said external optical surfaceselected to withstand a predetermined level of handling is operativewith at least one of a predetermined level of scratches, spills andfingerprints.
 4. The receiver according to claim 1, wherein saidtransparent optical impingement surface is the entrance window of saidat least one photovoltaic cell.
 5. The receiver according to claim 1,wherein said optical input element having an external optical surface isthe entrance window of a light guide, having said at least onephotovoltaic cell disposed at an end of said light guide remote fromsaid transparent optical impingement surface.
 6. The receiver accordingto claim 1, wherein said external optical surface comprises either aglass produced to have a high resistance to mechanical damage or a glasshaving a scratch resistant coating thereupon.
 7. The receiver accordingto claim 1, wherein said predetermined level of handling is determinedaccording to the expected effects of domestic use of said receiver. 8.The receiver according to claim 1, wherein said minor part of the areaof the input aperture is such that the major part of said incident beamof optical power is absorbed by said receiver.
 9. The receiver accordingto claim 1, wherein said retroreflector covering a minor part of thearea of the input aperture causes said reflection to diverge backtowards said transmitter, even if said incident beam is essentiallycollimated.